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Why Mount St. Helens was such a shocking event

Worries about another eruption have grown as scientists have detected a swarm of earthquakes under the mountain.

A new study indicates that there has been a swarm of earthquakes underneath Mount St. Helens since March, but scientists have said they don’t think this is an indication an eruption is imminent — and that’s a good thing, considering what happened during the eruption of 1980.

Mount St. Helens blew its top almost exactly 36 years ago to the day, killing 57 people and flattening 250 homes, sending a huge debris avalanche that registered 5.1 on the Richter scale. It also wiped out 47 bridges, 15 miles of railways, and 185 miles of highway. The height of the mountain itself shrunk from 9,677 to 8,363 feet, and was left with a horseshoe-shaped crater.

It was such a shocking event because no one had ever witnessed the destructive power of a volcano in the United States before. We’ve long known how destructive volcanoes can be, and certainly legendary tales of Mount Vesuvius and Pompeii shows why we never want to be around one, but until Mount St. Helens, the United States had never experienced an eruption of that size, and it caught many people near the mountain totally unaware.

The disaster could have been much worse. If it had happened one day later, loggers would have been at work. Since it happened on a Sunday, it killed mostly people who were observing the activity in the mountain, thinking they were a safe distance when in reality they were in the danger zone.

“At the same time as the earthquake, the volcano’s northern bulge and summit slid away as a huge landslide—the largest debris avalanche on Earth in recorded history,” the USGS said in a statement on the 1980 eruption. “A small, dark, ash-rich eruption plume rose directly from the base of the debris avalanche scarp, and another from the summit crater rose to about 200 m (650 ft) high. The debris avalanche swept around and up ridges to the north, but most of it turned westward as far as 23 km (14 mi) down the valley of the North Fork Toutle River and formed a hummocky deposit. The total avalanche volume is about 2.5 km3 (3.3 billion cubic yards), equivalent to 1 million Olympic swimming pools.

“The lateral blast devastated an area nearly 30 km (19 mi) from west to east and more than 20 km (12.5 mi) northward from the former summit,” the statement added. “In an inner zone extending nearly 10 km (6 mi) from the summit, virtually no trees remained of what was once dense forest. Just beyond this area, all standing trees were blown to the ground, and at the blast’s outer limit, the remaining trees were thoroughly seared. The 600 km2 (230 mi2) devastated area was blanketed by a deposit of hot debris carried by the blast.”